Computational and Experimental IM-MS Determination of the Protonated Structures of Antimalarial Drugs

A combination of ion mobility-mass spectrometry (IM-MS) measurements and computational methods were used to study structural and physicochemical properties of a range of quinoline-based drugs: amodiaquine (AQ), cinchonine (CIN), chloroquine (CQ), mefloquine (MQ), pamaquine (PQ), primaquine (PR), quinacrine (QR), quinine (QN), and sitamaquine (SQ). In experimental studies, ionization of these compounds using atmospheric pressure chemical ionization (APCI) yields monoprotonated species in the gas phase while electrospray ionization (ESI) also produces diprotonated forms of AQ, CQ, and QR and also for PQ, SQ, and QN in the presence of formic acid as an additive. Comparison of the trajectory-method-calculated and experimental IM-derived collisional cross sections (CCSN2) were used to assign both the protonation sites and conformer geometry of all drugs considered with biases of 0.7–2.8% between calculated and experimental values. It was found that, in solution, AQ and QR are protonated at the ring nitrogen of the quinoline group, whereas the other drugs are protonated at the amine group of the alkyl chain. Finally, the conformers of [M + H]+ and [M + 2H]2+ assigned according to the lowest energies and CCSN2 calculations were used to calculate the pKa values of the antimalarial drugs and the relative abundance of these ions at different pH values that provided validation of the computational and experimental IM-MS results.


Figure S12 .
Figure S12.Optimized structures of protomers and conformers of mono-protonated Amodiaquine, [AQ+H] + , in the gas phase.Letters a, b, and c indicate different sites of protonation.

Figure S13 .
Figure S13.Optimized structures of protomers and conformers of mono-protonated Chloroquine, [CQ+H] + , in the gas phase.Letters a, b, and c indicate different sites of protonation.

Figure S14 .
Figure S14.Optimized structures of protomers and conformers of mono-protonated Quinacrine, [QR+H] + , in the gas phase.Letters a, b, and c indicate different sites of protonation.

Figure S15 .
Figure S15.Optimized structures of protomers and conformers of mono-protonated Pamaquine, [PQ+H] + , in the gas phase.Letters a, b, and c indicate different sites of protonation.

Figure S16 .
Figure S16.Optimized structures of protomers and conformers of mono-protonated Primaquine, [PR+H] + , in the gas phase.Letters a, b, and c indicate different sites of protonation.

Figure S17 .
Figure S17.Optimized structures of protomers and conformers of mono-protonated Sitamaquine, [SQ+H] + , in the gas phase.Letters a, b, and c indicate different sites of protonation.

Figure S18 .
Figure S18.Optimized structures of protomers and conformers of mono-protonated Quinine, [QN+H] + , in the gas phase.Letters a, b, and c indicate different sites of protonation (c indicates protonation at OH group).

Figure S19 .
Figure S19.Optimized structures of protomers and conformers of mono-protonated Cinchonine, [CQ+H] + , in the gas phase.Letters a, b, and c indicate different sites of protonation (c indicates protonation at OH group).

Figure S20 .
Figure S20.Optimized structures of protomers and conformers of mono-protonated mefloquine, [MQ+H] + , in the gas phase.Letters a and b indicate different sites of protonation.

Figure S21 .
Figure S21.APCI-mass spectra of (a) CIN and (b) QN with CID voltages of 10 and 20 V, respectively.

Table S2 .
The calculated relative Gibbs free energies of different protomers and conformers of [AQ+H] + in the gas phase and in aqueous solution, their relative abundances in solution, and their calculated CCS N2 values.(Experimental DT CCS N2 = 193.7 and 186.1 Å 2 and computed Boltzmann-weighted CCS N2 =192.7 Å 2 ).

Table S3 .
The calculated relative Gibbs free energies of different protomers and conformers of [CQ+H] + in the gas phase and in aqueous solution, their relative abundances in solution, and their calculated CCS N2 values.(Experimental DT CCS N2 = 175.3Å 2 and computed Boltzmann-weighted CCS N2 =173.1 Å 2 ).

Table S4 .
The calculated relative Gibbs free energies of different protomers and conformers of [QR+H] + in the gas phase and in aqueous solution, their relative abundances in solution, and their calculated CCS N2 values.(Experimental DT CCS N2 = 195.3Å 2 and computed Boltzmann-weighted CCS N2 =192.0Å 2 )

Table S5 .
The calculated relative Gibbs free energies of different protomers and conformers of [PQ+H] + in the gas phase and in aqueous solution, their relative abundances in solution, and their calculated CCS N2 values.(Experimental DT CCS N2 = 175.6Å 2 and computed Boltzmann-weighted CCS N2 =175.5 Å 2 )

Table S6 .
The calculated relative Gibbs free energies of different protomers and conformers of [PR+H] + in the gas phase and in aqueous solution, their relative abundances in solution, and their calculated CCS N2 values.(Experimental DT CCS N2 = 161.7 Å 2 and computed Boltzmann-weighted CCS N2 =160.4Å 2 ).

Table S7 .
The calculated relative Gibbs free energies of different protomers and conformers of [SQ+H] + in the gas phase and in aqueous solution, their relative abundances in solution, and their calculated CCS N2 values.(Experimental DT CCS N2 = 182.9Å 2 and computed Boltzmann-weighted CCS N2 =180.0Å 2 ).

Table S9 .
The calculated relative Gibbs free energies of different protomers and conformers of [CIN+H] + in the gas phase and in aqueous solution, their relative abundances in solution, and their calculated CCS N2 values.(Experimental DT CCS N2 = 166.8,169.8 and 176.2 Å 2 and computed Boltzmann-weighted CCS N2 =166.5 Å 2 ).

Table S10 .
The calculated relative Gibbs free energies of different protomers and conformers of [MQ+H] + in the gas phase and in aqueous solution, their relative abundances in solution, and their calculated CCS N2 values.(Experimental DT CCS N2 = 183.4Å 2 and computed Boltzmann-weighted CCS N2 =179.2Å 2 ).

Table S11 .
The calculated relative Gibbs free energies of different conformers of [AQ+2H] 2+ in the gas phase and in aqueous solution and their calculated CCS N2 values.(Experimental DT CCS N2 = 237.8Å 2 ).

Table S12 .
The calculated relative Gibbs free energies of different conformers of [CQ+2H] 2+ in the gas phase and in aqueous solution and their calculated CCS N2 values.(Experimental DT CCS N2 = 236.4Å 2 ).

Table S14 .
The calculated relative Gibbs free energies of different conformers of [PQ+2H] 2+ in the gas phase and in aqueous solution and their calculated CCS N2 values.(Experimental DT CCS N2 = 234.1 Å 2 ).

Table S15 .
The calculated relative Gibbs free energies of different conformers of [PR+2H] 2+ in the gas phase and in aqueous solution and their calculated CCS N2 values.